Diamond and graphite, as crystal structures of carbon, have been well known in the field. C60 was found in 1985 by R. E. Smalley, R. F. Curl and H. W. Kroto et al. (for example, Nature, 318: 162-163, 1985), and has a soccer ball-like structure comprising 12 pentagons and 20 hexagons. In addition to C60, there are other large basket-like molecules such as C70 and C76. This series of molecules is called “fullerene.” Carbon compounds with structures that were previously unknown, such as “carbon nanotube” (Nature, 354: 56-58, 1991; Japanese Laid-Open Patent Application No. 2001-64004) and “carbon nanohorn” (Chem. Phys. Lett., 309: 165-170, 1999; Japanese Laid-Open Patent Application No. 2001-64004), were successively discovered by Sumio Iijima, in 1991 and 1999, respectively. All of these fullerenes, carbon nanotubes and carbon nanohorns comprise six- and five-membered rings of carbon atoms, and form nanometer-scale fine structures; therefore, they have recently attracted a lot of attention as “nanographite structures”.
There are many reasons that nanographite structures are of particular interest in the field. For example, “carbon nanotubes can have both properties of metal and semiconductor due to the difference in their chirality” (Nature, 391: 59-62), and “metal-doped fullerene exhibits superconductivity” (Nature, 350: 600-601). Furthermore, nanographite structures attract attention because of the “selective gas storage capability shown by carbon nanohorns” (Nikkei Science, 42, August issue, 2002), the “ability of carbon nanohorn for the support and sustained release of pharmaceutical compounds” (Japanese Patent Application No. 2004-139247; Mol Pharmaceutics 1: 399), and the like. With the use of these characteristic properties, nanographite structures may be applied to new electrical materials, catalysts, optical materials, and other fields; in particular, they may be used for wiring of semiconductors, fluorescent indicator tubes, fuel cells, gas storage, vectors for gene therapy, cosmetics, drug delivery systems, biosensors, etc.
The present inventors and others have isolated a peptide motif which binds to a carbon nanohorn, one of nanographite structures, by the phage display technique (Japanese Laid-Open Patent Application No. 2004-121154; Langmuir, 20, 8939-8941, 2004).
On the other hand, ferritin proteins have been well known as a protein which stores “molecules of ‘iron,’ which is an essential metal and is toxic at the same time” in living bodies. Ferritin exists universally, from animals and plants to bacteria, and is deeply involved in the homeostasis of iron element in living bodies or in cells. Ferritin from higher eukaryotes such as human and horse forms a spherical shell structure consisting of a 24-mer approximately 12 nm in diameter, formed from peptide chains whose molecular weight is about 20 kDa, and has an interior space of 7 to 8 nm. Ferritin stores iron molecules in this interior space as a mass of nanoparticulate iron oxide. With regard to 24 subunits which constitute a protein spherical shell (cage), there are two types (type H and type L), and the ratio of these types varies depending on organism species and tissues.
Ferritin stores iron nanoparticles inside it under natural circumstances. However, under artificial circumstances, it has been revealed that ferritin can store the substances in addition to iron such as oxides of beryllium, gallium, manganese, phosphorus, uranium, lead, cobalt, nickel, chromium, etc., and nanoparticles of semiconductors, magnets such as cadmium selenide, zinc sulfide, iron sulfide and cadmium sulfide. Consequently, applied research of ferritin in the fields of material engineering of semiconductors and health care has been actively conducted.
If it is possible to combine nanographite structures having excellent properties with metal-filled ferritin molecules, the development of composite materials having an unprecedented new function can be expected. In this case, a technique for making ferritin molecules efficiently recognize and bind to nanographite structures such as carbon nanotubes and carbon nanohorns, is required.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.